Identification of genes contributing to common (non-Mendelian) forms of breast cancer would enable early identification of individuals at risk and tailoring of prevention and treatment regimens. This is an extremely difficult problem since heritable risk for common forms of breast cancer is a result of multiple gene-gene interactions, with each individual gene making only a minor contribution to risk. Standard approaches (e.g. genetic association studies) are underpowered to detect the many gene-gene interactions underlying risk. We propose a phenotype-based approach to predict gene-gene interactions underlying risk for common forms of breast cancer. The approach is based on the observation that defects in the cellular DNA damage response (an intermediate phenotype for breast cancer susceptibility) are heritable, making it possible to map gene-gene interactions responsible for this phenotype. We hypothesize that gene-gene interactions governing sensitivity to DNA damage are conserved between yeast and humans. Our approach will be to: i) use the yeast S. cerevisiae to perform genome-wide screens for mutations that synergize with homologs of human breast cancer tumor suppressor genes to confer sensitivity to DNA damage, and ii) to screen amongst these identified synthetic interactions for those that are conserved in human mammary epithelial cells. Specific Aim 1: Perform genome-wide screens in S. cerevisiae to identify mutations that synergize with yeast homologs of human breast cancer tumor suppressor genes to produce sensitivity to DNA damage. Specific Aim 2: Using RNA interference, build human mammary epithelial cell lines that have stable knock- down of breast tumor suppressor genes. Specific Aim 3: Determine if genetic interactions discovered in yeast are conserved in human breast epithelial cells. PROJECT NARRATIVE: Determination of an individual's risk for developing cancer could have tremendous public health impact in several ways: i) identifying risk alleles for breast cancer will facilitate cost-effective cancer screening programs tailored to an individual's risk profile;ii) identifying risk alleles will allow identification of women who will most benefit from prevention strategies;iii) identifying risk alleles will elucidate the pathophysiology of the disease, leading to targeted therapeutics or chemoprevention agents such as existing targeted hormonal and antibody therapies.